1. Field of the Invention
The present invention relates to a method of obtaining a powdery material from a solution of a material to be dried.
2. Description of the Related Art
Various methods have been proposed in the past to obtain a powdery material, for example, powdery polymerizate from, a solution of a material to be dried, for example, polymerizate. For example, in one method using a centrifugal thin film evaporator, a polymerizate is extracted in the molten state and then cooled and solidified. In another method, the solution is coated in the form of a sheet on a belt in a band drier, and a dry powder is obtained. The former method, however, has the disadvantages that the viscosity rises sharply so that the material cannot be drawn off, and as operations are carried out at high temperature, deteriorations occur which are undesirable from the viewpoint of quality. The latter method, on the other hand, suffers from the disadvantage that due to a sharp rise in viscosity, the volatile constituents cannot be completely evaporated.
In addition, in another conventional method known as reprecipitation, a solution of the polymerizate is dripped into a solvent which is a poor solvent for the polymerizate but a good solvent for the solvent in which the polymerizate is dissolved. This precipitates the polymerizate, which is then filtered and dried. As this method requires a very large amount of solvent, however, it requires a large tank and productivity therefore declines. Further, the mother liquors from filtration must also be treated, and the method is thus not economical.
Another method uses a heated long tube as described in, for example, JP A-58-79501 and JP-A 60-90001 ("JP-A-" means Unexamined Published Japanese Patent Application). In this method, the solution containing a volatile component and a component which is difficult to volatize is evaporated in a heated evaporator, and supplied continuously to a cooling crusher. In the crusher, the component which is difficult to volatize is cooled, solidified and crushed while volatile components remaining in said non volatile components are further evaporated.
There are, however, various problems in attempting to dry a polymerizate solution by means of a heated long tube. When polymerizate solution supplied at a constant flow rate is blown from said tube into a reduced pressure vessel, the product obtained is often a sticky solution containing 10-20 wt % of residual volatile components, or a massive material which contains sticky solution and is not yet dry. Even if the operating conditions and equipment conditions are varied, it only produces a slight change in the volatile components, and a dry material that can be handled as a powder is not easily obtained. Furthermore, if the product of blowing is a sticky solution and a massive material containing a sticky solution, it is extremely difficult to remove it from the reduced pressure vessel to a condition under atmospheric pressure.
To blow a polymerizate as powdery state from a heated long tube is thus an important issue.
Further, the cooling crusher has a complex structure and is costly, and if it is attempted to produce many different types of powders with one apparatus, its complex structure makes cleaning difficult when changing over from one product to the other product. In particular, it was found that this problem constitutes a considerable obstacle industrially when the non-volatile component is the objective product.
According to past experience, when the long tube was heated, hot water is used as heating medium if a temperature of 100.degree. C. or below was desired, or steam is used if the temperature was 100.degree. C. or above. An example of the former process is disclosed in JP-B-61-14777 ("JP-B-" means examined Japanese patent application), and an example of the latter in JP-A-58-79501 and JP-A-6090001. If said methods are applied to a polymerizate of low softening point, however, the heat capacity of hot water is insufficient, and then only a thick sticky liquid can be obtained. If on the other hand, steam at 100.degree. C. or over is used as in the latter method, a powdery polymerizate is blown out. As this has a low softening point, however, it is soft, and the powder sticks together to form lumps or adhere to the wall of the reduced pressure vessel, and cannot be drawn off.